The human gut is the natural habitat for a diverse and dynamic microbial ecosystem having an important impact on health and disease. Bacteria have lived in and on animal hosts since multicellular life evolved about 1 billion years ago. Hosts provide habitat and nutrition to the microbial communities and derive many benefits from their guests that contribute with metabolic (recovery of energy and nutrients), defensive (barrier effect against invaders) and trophic (immune regulation, neuro-endocrine development) functions. Several disease states or disorders have been associated with changes in the composition or function of the enteric microbiota, including inflammatory bowel diseases, obesity and the metabolic syndrome. Probiotics and prebiotics can be used to improve symbiosis between enteric microbiota and host, or correct states of dysbiosis. Table of Contents: Microbial Communities / Host-Microbe Interactions in the Gut / Composition of the Human Enteric Microbiota / Acquisition of the Enteric Microbiota / Dysfunction of the Enteric Microbiota / Therapeutic Manipulation of the Enteric Microbiota / References / Author Biography